1. Field of the Invention
The present invention relates generally to a fabrication process for a magneto-resistive effect device of the CPP structure, and more particularly a fabrication process for a Heusler alloy layer that constitutes a part of the multilayer structure of that device.
2. Explanation of the Prior Art
With recent improvements in the plane recording density of hard disk systems, there has been growing demands for improvements in the performance of thin-film magnetic heads. For the thin-film magnetic head, a composite type thin-film magnetic head has been widely used, which has a structure wherein a reproducing head having a read-only magneto-resistive effect device (hereinafter often referred to as the MR (magneto-resistive) device for short) and a recording head having a write-only induction type magnetic device are stacked on a substrate.
For the MR device, there is the mention of an AMR device harnessing an anisotropic magneto-resistive effect, a GMR device harnessing a giant magneto-resistive effect, a TMR device harnessing a tunnel-type magneto-resistive effect, and so on.
The reproducing head is required to have high sensitivity and high outputs in particular. GMR heads using a spin valve type GMR device have already been mass-produced as a reproduction head possessing such performances, and to meet further improvements in plane recording densities, reproducing heads using TMR devices are now being mass-produced, too.
In general, the spin valve type GMR device comprises a nonmagnetic layer, a free layer formed on one surface of that nonmagnetic layer, a fixed magnetization layer formed on another surface of the nonmagnetic layer, and a pinned layer (generally an antiferromagnetic layer) on the side of the fixed magnetization layer facing away from the non-magnetic layer. The free layer has its magnetization direction changing depending on an external signal magnetic field, and the fixed magnetization layer has its magnetization direction fixed by a magnetic field from the pinned layer (antiferromagnetic layer). In a preferable embodiment of the fixed magnetization layer, it is made up of a synthetic pinned layer with a nonmagnetic intermediate layer sandwiched between an inner pin layer and an outer pin layer.
Incidentally, common GMR heads used so far in the art have a CIP (current in plane) structure wherein a current for detecting magnetic signals (the so-called sense current) is passed parallel with the plane of each of the layers forming the GMR device. On the other hand, GMR devices having the so-called CPP (current perpendicular to plane) structure wherein the sense current is passed perpendicularly (stacking direction) to the plane of each of the layers forming the GMR device, too, are now under development as next-generation ones. The aforesaid TMR devices, too, would come under the CPP structure category.
In the GMR devices proposed so far in the art, the free layer and fixed magnetization layer are still composed mainly of CoFe alloys, NiFe alloys or the like. Referring to such GMR devices of the CPP structure, even when they have a multilayer structure capable of achieving practical reproduction gap lengths, the magneto-resistivity change ratio (MR ratio)—the ratio of a magneto-resistivity change with respect to resistance—is barely about 4%, a figure still practically less than satisfactory. A possible reason why the MR ratio of conventional GMR devices of the CPP structure is small could be that the spin polarizability of CoFe or NiFe alloys used as the material for the free layer and fixed magnetization layers is small.
To increase the MR ratio of the GMR devices of the CPP structure, it has recently been proposed to use as the material for the free layer and fixed magnetization layer a Heusler alloy that is a sort of half-metal with its spin polarizability close to 1 (JP-A's 2005-51251 and 2005-116701).
Heusler alloy layers are generally formed by means of sputtering techniques.
However, inventors' studies have now revealed that for the reason that a Heusler alloy target has the nature of being so fragile that it can break up, sputtering at an increased input power offers a problem in that cracks appear in the target itself. It is thus still impossible to bring up the film-deposition rate, failing to boost productivity, and the ensuing film quality is far away from the expected level as well.
The situations being like this, the present invention has for its object to provide a Heusler alloy layer formation process by which the film-deposition rate can be brought up with improvements in productivity and device performance as well as a fabrication process for a magneto-resistive effect device of the CPP structure, by which productivity and device performance can be improved.